1. Field of the Invention
The invention relates to floating offshore vessels. More specifically, the invention relates to floating offshore vessels designed for at least the storage of hydrocarbon products.
2. Description of Related Art
A Floating Production, Storage and Offloading vessel (FPSO; also called a “unit” and a “system”) is a type of floating tank system used by the offshore oil and gas industry and designed to take all of the oil or gas from nearby oil and gas wells, process it, and store it until the oil or gas can be offloaded onto waiting tankers, or sent through a pipeline. The temporary storage allows production from subsea operations to accumulate until a sufficient quantity is available to offload to a tanker for transportation to the mainland.
A number of different designs with various advantages and disadvantages populate the industry. Some FPSO structures are decommissioned tankers that are suitable for large fixed storage. Some FPSO structures are designed to be submerged partially below the water surface as a semi-submersible structure. One advantage of semi-submersible units for oil storage is lowering the pitch and roll from wave action by having more structure below the surface. For such structures, it is generally known to support a working deck above the water surface when the semi-submersible is at its lowest normal submersion level. Such FPSOs traditionally have at least partially submerged the storage tanks for oil and built columnar supports from the tanks upward to support the deck above the water surface.
However, recent efforts in the oil and gas industry have focused on liquefied natural gas (LNG) with particular requirements. LNG is natural gas that has been converted to liquid form for ease of storage or transport. The liquefaction process involves removal of certain components (such as dust, helium, or impurities that could cause difficulty downstream, e.g. water, and heavy hydrocarbons) and then condensed into a liquid at close to atmospheric pressure by cooling it to cryogenic temperatures. LNG is transported in specially designed cryogenic sea vessels and stored in specially designed tanks. LNG is about 1/614th the volume of natural gas at standard temperature and pressure (STP), making it much more cost-efficient to transport over long distances where pipelines do not exist. Currently, common tank types are membrane (TGZ Mark III and GT96) and Moss Rosenberg (spheres) or Self-Supporting Prismatic Type. Among them, membrane-type LNG tanks are most widely used because of its lower material and fabrication cost. However, it is a general consensus that membrane-type LNG tanks may not sustain sloshing impact load at partial filling condition unless roll motions of the carrier are very small, i.e. less than 5 degrees. LNG FPSOs are needed to operate at all filling levels. Ship-type FPSOs have significant roll motion response in waves and are ill-suited to fulfill the requirement for small roll motion in most of the potential installation sites for LNG FPSOs.
In addition to the sloshing issue, there is a stricter requirement on motion response of LNG FPSO regarding LNG liquefaction plant to be placed on the topsides of the FPSO. Most of the well-proven high efficiency LNG liquefaction technology requires heel angle of the hull as small as 2 degrees during the FPSO operation. The existing designs of ship-type and semi-submersible FPSOs can hardly meet these criteria.
FPSO hulls for LNG applications, especially for membrane-type LNG tanks, also can have higher structural requirements as compared to oil-storage application. The structural integrity of insulation system structures and connections between insulation systems and hulls is sensitive to the local deflection and vibration of the tank hull. It is highly desirable that possible major structural loads, such as vertical shear force and bending moment due to topside weight and wave load, are not directly transferred to the tank wall structures.
Although less critical than the requirements on hull motion and structural integrity, capability of float-over installation for topsides will be another desirable aspect of LNG FPSO hull. In case of a ship-type FPSO, the high freeboard of FPSO limits use of float-over installation of topsides, which results in longer construction periods in dry dock and quayside. In case of semi-submersible types, spacing between columns needs to be wide enough for installation vessels to pass through. Further, enough ballasting capacity is needed to lower the freeboard to the required level during the installation.
To provide safer storage of LNG cargo and more flexible options for construction, installation and operation of LNG facilities, a new hull design is needed that can provide smaller motion response in waves and smaller footprints on and above the mean water level. It is also desirable for the new design that the hull shape and arrangement should be able to provide sufficient structural integrity to support loads above the storage area without interfering with LNG tank structures.